This application relates generally to gas turbine engines and, more particularly, to a flowpath through a blisk rotor assembly.
A gas turbine engine typically includes at least one rotor including a plurality of rotor blades extending radially outwardly from a common annular rim. Specifically, in blisk rotors, the rotor blades are formed integrally with the annular rim rather than attached to the rim with dovetail joints. An outer surface of the rim typically defines a radially inner flowpath surface for air flowing through the rotor assembly.
Centrifugal forces generated by the rotating blades are carried by portions of the rims below the rotor blades. The centrifugal forces generate circumferential rim stress concentration between the rim and the blades. Additionally, a thermal gradient between the rim and the rotor disk during transient operations generates thermal stresses which may adversely impact a low cycle fatigue life of the rotor assembly. Also, because the rim is exposed directly to the flowpath air, thermal gradients and rim stress concentrations may be increased. Furthermore, as the rotor blades rotate, blade roots may generate local forces that may further increase the rim stress concentration.
To reduce the effects of circumferential rim stress concentration, additional material is provided at each root fillet to increase a radius of the root fillet. However, because the root fillets are exposed to the flowpath air, the additional material attached to the root fillets may be detrimental to flow performance.
Other known rotor assemblies include a plurality of indentations extending between adjacent rotor blades over an axial portion of the rims between the rim leading and trailing edges. The indentations are defined and formed as integral compound features in combination with the root fillets and rotor blades. Typically such indentations are formed using an electrochemical machining, ECM, process. Because of dimensional control limitations that may be inherent with the ECM process, surface irregularities may be unavoidably produced. Such surface irregularities may produce stress radii on the rim which may result in increased surface stress concentrations. The surface irregularities therefore are milled with hand bench operations. Such hand bench operations increase production costs for the rotor assembly. Furthermore, because such indentations extend to the rim trailing edge, a forward facing step is created for an adjacent downstream stator stage. Such steps may be detrimental to flow performance.
In an exemplary embodiment, a blisk rotor assembly includes an outer rim including a curved outer surface for facilitating a reduction in circumferential rim stress generated during engine operations. More specifically, in the exemplary embodiment, the rotor assembly includes a blisk rotor including a plurality of rotor blades and a radially outer rim. The rotor blades are integrally formed with the rim and extend radially outward from the rim. A root fillet provides support to rotor blade/rim interfaces and extends circumferentially around each rotor blade/rim interface between the rotor blade and rim. The rim includes an outer surface having a concave curved indentation extending between adjacent rotor blades. Each curved indentation extends from a leading edge of the rotor blade towards a trailing edge of the rotor blade and forms a compound radius. The compound radius includes a first radius and a second radius. The first radius is defined by a root fillet adjacent a pressure side of each rotor blade and the second radius is larger than the first radius and extends from the first radius. Each indentation is tapered to end within a portion of the outer rim between adjacent rotor blades.
During operation, as the rotor blades rotate, centrifugal loads generated by the blades are carried by portions of the outer rim below each rotor blade. As air flows between adjacent rotor blades, the outer rim facilitates a reduction in thermal gradients that may be generated between the rotor blades and the outer rim, thus reducing thermal stresses that could impact a low cycle fatigue life (LCF) of the rotor assembly in comparison to at least some other known rotor assemblies. The curved surface provides stress shielding and reduce stress concentrations by interrupting circumferential stresses below the rotor blade root fillets. Because the second radius is larger than the first radius, a lower stress concentration is generated in the circumferential stress field and less circumferential rim stress concentration is generated between the rim and the rotor blades in comparison to at least some other known rotor assemblies. As a result, the rotor assembly facilitates high efficiency operation and a reduction in circumferential rim stress concentration.